The quadrilateral quadratus lumborum forms a thick muscular sheet in the posterior abdominal wall. It lies adjacent to the lumbar transverse processes and is broader inferiorly. Close to the 12^th rib, the lateral arcuate ligament crosses the quadratus lumborum. The subcostal nerve passes posterior to this ligament and runs inferolaterally on the quadratus lumborum. Branches of the lumbar plexus run inferiorly on the anterior surface of this muscle.